(1) Technical Field
This invention relates generally to wafer transfer and handling apparatus and more particularly to a positioning gage that assures proper alignment between elements of the wafer transfer mechanism and the wafers being loaded to and from a vertical CVD/diffusion system in a semiconductor process.
(2) Description of the Prior Art
The following two documents relate to various methods dealing with handling of wafers during the production of semiconductor devices.
U.S. Pat. No. 3,730,595 issued May 1, 1973 to Carl Yakubowski discloses a wafer transfer and handling apparatus having an indexable carrier for transferring wafers to and from work stations.
U.S. Pat. No. 5,195,729 issued Mar. 23, 1993 to Michael E. Thomas et al discloses a carrier for a semiconductor wafer or other substrate has an outer portion adapted for engagement by equipment for processing wafers.
The manufacturing process of semiconductor wafers generally includes a CVD(Chemical Vapor Depostion)/diffusion treatment step in a CVD/diffusion furnace. A vertical CVD/diffusion furnace conventionally has a firing receptacle for wafers, a wafer handling apparatus and a supply of wafers arranged and stored in cassette compartments. Wafers must be removed and replaced many times during this process. With the existance of high throughput automatic process equipment, automatic wafer handling is most essential. Indexing of the elevator to the proper cassette address, the retrieval and transfer of wafers from the storage compartment into the firing receptacle of the furnace and conversely, requires precision movements and alignments to prevent damage to the fragile wafers. Wafer retrieval and transfer are subject to damaging wafers. The handling mechanism has five vacuum tweezers disposed vertically. The longitudinal distance between tweezers is the same pitch increment that separates the wafers in the cassette stocker and the firing receptacle. During the handling operation, the tweezers are introduced between the wafers for inserting or pulling them into or out of their respective holders. During the handling operation and after many cycles of use, the spacing between tweezers may change because of loose fasteners or damaged and displaced tweezer blades. When this comes about, mechanical interference between the handling mechanism and wafers causes damage to both product and equipment. The prior art has failed to provide means to detect or to prevent damage to wafers and to apparatus caused by tweezer misalignment.